refactor

vec.ts

@@ -38,9 +38,14 @@ export default class Vec2d implements Point {
 
   normalize(): Vec2d {
     const len = this.length();
+    if (len === 0) return new Vec2d(0, 0); // avoid division by zero
     return new Vec2d(this.x / len, this.y / len);
   }
 
+  neg(): Vec2d {
+    return new Vec2d(-this.x, -this.y);
+  }
+
   scale(scalar: number): Vec2d {
     return new Vec2d(this.x * scalar, this.y * scalar);
   }
@@ -61,6 +66,10 @@ export default class Vec2d implements Point {
     return this.sub(other).length();
   }
 
+  dot(other: Vec2d): number {
+    return this.x * other.x + this.y * other.y;
+  }
+
   lerp(other: Vec2d, t: number): Vec2d {
     // (1-t)*A + B*t
     return this.scale(1 - t).add(other.scale(t));